This invention relates to antennas for use in airborne automatic direction finding (ADF) equipment and particularly to such antennas which are relatively small but whose electrical characteristics are similar to those antennas which are relatively large.
Modern automatic radio direction finders for aircraft use an antenna which is fixed to the skin of the aircraft and which includes what are known as loop and sense elements. In this description the word "loop" is used to describe different elements in various arts. Specifically, in the ADF art, a loop element or antenna is a type of radiated signal responsive circuit which provides a portion of the signal input into ADF receiver. In the antenna art, specifically, a loop is a closed electrical circuit usually comprised of one or more turns of copper wire and which responds directionally to intercepted electromagnetic radiations. The ADF loop element or antenna is generally comprised of two orthogonally placed loops. Specifically, the ADF loop element comprises two mutually perpendicular electrical windings on a ferrite form or forms. The amplitudes of the signals induced in the various windings by an electromagnetic field of the type radiated by radio broadcast stations is dependent upon the orientation of the loop elements with respect to the broadcast station. By considering the amplitudes of the induced signals, the direction of the broadcast station from the loop elements can be ascertained with a 180 degree ambiguity. The ADF antenna also includes an omnidirectional sense antenna which provides phase information to resolve the ambiguity.
A generally larger loop was previously needed to obtain a sufficient signal-to-noise ratio over the ADF operating range, which at the present time lies between 200 kilohertz to 1.8 megahertz, since the signal-to-noise ratio is a function of the equivalent effective height of the loop and the noise generated by the circuit connected to the loop.
In known ADF the signals from the two loop antenna windings are modulated by a low frequency local signal and combined to produce a composite loop signal. This signal is added to the signal from the sense antenna, the new signal comprising the directional information. This signal can be demodulated to provide the directional information by various means known to those skilled in the art and which do not comprise a portion of the present invention. For example, a coherent demodulator for extracting the directional information from the combined signal was described in a patent application entitled "Coherent Demodulator" by Joseph J. Sawicki, having Ser. No. 805,676, now U.S. Pat. No. 4,135,191, issued Jan. 16, 1979, and filed June 13, 1977.
It is important in preserving the direction information, when the sense and loop signals are added, that these signals have a relatively constant phase difference of 90 degrees over the ADF operating frequency range.